This invention relates generally to balancing automotive vehicle wheels and more particularly, to methods and systems for balancing automotive vehicle wheels with rims that are constructed with radially extending, spaced apart spokes.
Many automotive wheel balancer manufacturers offer a feature which balances spoked wheels where an imaginary vertical “plane” perpendicular to the wheel axis of rotation and located directly behind the spokes of the wheel is selected for application of balance correction weights. Spoke locations are made known to the balancer by user manual input or via optical methods as described by US2005/0052658 to Braghiroli. After the wheel imbalance is measured and the required angular placement is known for the required correction weight at the spoke plane, vector equivalent weights are resolved and displayed for placement behind two spokes. This feature is typically referred to as a “split” function.
U.S. Pat. No. 5,355,729 to Douglas describes a generic “split weight” function using incremental weights for the split with variable placement angle, U.S. Pat. No. 5,591,909 to Hofmann describes fixed placement angles for the split (two spokes) and variable weight magnitudes (rounded to increments), and U.S. Pat. No. 4,357,832 to Blackburn describes a turbine blade balancing procedure where fixed placement angles are provided (the blades) and variable weight removal from the blades accomplishes the “split” balance of the rotor. For automatic wheel balancing, spoke plane “split” functions are predominately applied during a “two plane (inner and outer) dynamic balance” mode where two weights simultaneously correct for both static imbalance (shake force due to radial off-center of mass) and couple imbalance (twist forces caused only during rotation). The inner plane weight can be a clip-on style, adhesive style, or even a balance patch on the inside periphery of the tire. Static single plane balance mode is also sometimes used, but for that case the single plane weight can be a clip-on style on the inside rim lip, an adhesive style near the center of the rim where it is too far behind spokes to make any “split” function effective (one would still see the split weights when not viewing the wheel exactly straight on), or a patch style inside the tire (which is not applicable to rim spokes).
Although the automotive wheel “split” function provides a way to hide weights on spoked wheels from view, it causes extra labor for the operator and increases the chance of weight placement error because two weights must be applied to the weight plane located behind the spokes. Three weights in total are added to the wheel instead of the usual two weights. Italian Patent No. IT1319713 to Buzzi addresses these problems by recognizing that the desired placement angle for the “unsplit” weight on the spoke plane is seldom located exactly between spokes. Analysis of placing a weight only at the spoke nearest the un-split vector reveals that the residual imbalance error caused on the “spoke plane” by applying weight at only one spoke can be less than a predetermined imbalance limit for the spoke plane, enabling the possibility to balance a spoked wheel using only one spoke weight placement instead of two. But there are limitations with this method.
The computations for the single spoke weight magnitude and the “residual error” after applying this weight are only performed at the spoke plane, limiting how often the single spoke method can be used. Italian Patent No. IT1319713 to Buzzi acknowledges that when the un-split location is not near enough to a spoke, it is necessary to revert back to a “conventional” split method where weights behind two spokes are employed. For cases when the method can be used, the residual imbalance has a static component which could stack up with other residual static imbalances due to user weight placement error and wheel centering error. The importance of minimizing static imbalance is well documented in for example, U.S. Pat. No. 4,854,168 to Hofmann, U.S. Pat. No. 4,891,981 to Schenck, U.S. Pat. No. 5,915,274 to Douglas, and U.S. Pat. No. 5,689,069 to Corghi. A method of minimizing static imbalance is needed for all instances of wheel assembly balancing without causing any residual static imbalance and without restricting the method to being used only for certain imbalance conditions and spoke arrangements.